1. Field of the Invention
The present invention is generally directed toward manufacturing methods and, more particularly, toward a method for setting a clearance between a rocker arm adjustment screw and an engine valve.
2. Description of Related Art
Tappet clearance is the distance between a bottom surface of an adjustment or tappet screw and an upper surface of a valve. Accurately setting the tappet clearance, which is vital to proper operation of the engine, is time-consuming and labor-intensive. Unfortunately, when the cylinder head is secured on the engine block, the position of the adjustment screws, which much be adjusted to set the tappet clearance, prevents direct visual observation of the tappet clearance.
Accordingly, it is a common practice for an assembler to walk along and manually adjust the tappet clearance with feeler gauges as the engine travels on the assembly line. Unfortunately, this method results in non-repeatable and inconsistent results as the skill of each assembler directly affects the quality of the finished product. Moreover, tilting of the feeler gauges, as may occur in the manual tappet clearance setting method, will naturally produce inconsistent results.
In response to this problem, several solutions have been proposed. One solution is embodied in expired U.S. Pat. No. 3,988,925, the disclosure of which is expressly incorporated herein by reference in its entirety. In the '925 patent, a method for adjusting valve lash or adjustment screw clearance includes positioning the crank shaft in a neutral position, insuring that the valve is closed, turning the adjustment screw toward the valve and engaging the valve stem, and sensing movement of the valve retainer indicative of the valve opening to establish a zero reference. Once the zero reference is established, the direction of rotation of the adjustment screw is reversed until the adjustment screw is at the zero reference. Thereafter, the adjustment screw is reversed further away from the valve a predetermined amount to provide a desired clearance between the screw and the valve stem.
While the method taught in the '925 patent is a good starting point, it suffers from a number of disadvantages. Most importantly, it fails to account for many variables and tolerances present in the mechanical system that are causes for error in the tappet clearance. Accordingly, while results should, theoretically, be repeatable and consistent using the '925 method, in reality the results are inconsistent and provide a source of concern.
In practice, the variables or tolerances that result in tappet clearance errors when using the '925 method are numerous. For example, there is the clearance between the adjustment screw and the valve. This is the clearance that is to be determined and set to a predetermined value. There is also the clearance between the valve and the head. There is the further clearance between the rocker arm and the cam shaft. In the '925 patent, these two last mentioned clearances are assumed to be zero. Unfortunately, these clearances are not zero, are not consistent from engine to engine, and are a source of error which cause inconsistent and non-repeatable results.
Moreover, other variables or sources of error exist in the mechanical system under consideration. There exists a journal clearance between the rocker arm and the rocker shaft. There is also lash (lost motion) in the threaded connection between the screw and the rocker arm. This lost motion can be best understood by visualizing the screw threads engaging one edge of the rocker arm threaded opening when loaded and engaging the opposite edge when not loaded. This vertical displacement is a source of error. There is clearance in the connection between the adjustment screw and the adjustment screw bit, creating lost motion or lash when the direction of bit/screw rotation is reversed. The spring biased valve is under tension when at-rest and under compression during the opening/closing stroke, creating a measurable deflection of tension that is a source of error. There is also clearance between the valve guide and the valve stem, creating initial instability in the tracking of valve retainer and valve movement. Again, there is no means in the system taught by the '925 patent to account for these numerous potential sources of error which may vary from engine to engine. Accordingly, the results derived using the '925 method are less consistent and repeatable than desired.
Therefore, there exists a need in the art for a tappet clearance setting method that provides consistent, repeatable results. There also exists a need in the art for reducing or minimizing the effects of the various sources of error on the adjustment screw clearance.